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<title>Exceptions (Using the GNU Compiler Collection (GCC))</title>

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<h3 class="section" id="Exceptions-1"><span>8.7 Exceptions<a class="copiable-link" href="#Exceptions-1"> &para;</a></span></h3>

<p>GNU Objective-C provides exception support built into the language, as
in the following example:
</p>
<div class="example smallexample">
<pre class="example-preformatted">  @try {
    ...
       @throw expr;
    ...
  }
  @catch (AnObjCClass *exc) {
    ...
      @throw expr;
    ...
      @throw;
    ...
  }
  @catch (AnotherClass *exc) {
    ...
  }
  @catch (id allOthers) {
    ...
  }
  @finally {
    ...
      @throw expr;
    ...
  }
</pre></div>

<p>The <code class="code">@throw</code> statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a <code class="code">@catch</code> block, the
<code class="code">@throw</code> may appear without an argument (as shown above), in
which case the object caught by the <code class="code">@catch</code> will be rethrown.
</p>
<p>Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme.  When an object is thrown, it will be caught
by the nearest <code class="code">@catch</code> clause capable of handling objects of
that type, analogously to how <code class="code">catch</code> blocks work in C++ and
Java.  A <code class="code">@catch(id &hellip;)</code> clause (as shown above) may also
be provided to catch any and all Objective-C exceptions not caught by
previous <code class="code">@catch</code> clauses (if any).
</p>
<p>The <code class="code">@finally</code> clause, if present, will be executed upon exit
from the immediately preceding <code class="code">@try &hellip; @catch</code> section.
This will happen regardless of whether any exceptions are thrown,
caught or rethrown inside the <code class="code">@try &hellip; @catch</code> section,
analogously to the behavior of the <code class="code">finally</code> clause in Java.
</p>
<p>There are several caveats to using the new exception mechanism:
</p>
<ul class="itemize mark-bullet">
<li>The <samp class="option">-fobjc-exceptions</samp> command line option must be used when
compiling Objective-C files that use exceptions.

</li><li>With the GNU runtime, exceptions are always implemented as &ldquo;native&rdquo;
exceptions and it is recommended that the <samp class="option">-fexceptions</samp> and
<samp class="option">-shared-libgcc</samp> options are used when linking.

</li><li>With the NeXT runtime, although currently designed to be binary
compatible with <code class="code">NS_HANDLER</code>-style idioms provided by the
<code class="code">NSException</code> class, the new exceptions can only be used on Mac
OS X 10.3 (Panther) and later systems, due to additional functionality
needed in the NeXT Objective-C runtime.

</li><li>As mentioned above, the new exceptions do not support handling
types other than Objective-C objects.   Furthermore, when used from
Objective-C++, the Objective-C exception model does not interoperate with C++
exceptions at this time.  This means you cannot <code class="code">@throw</code> an exception
from Objective-C and <code class="code">catch</code> it in C++, or vice versa
(i.e., <code class="code">throw &hellip; @catch</code>).
</li></ul>

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